Traditional stethoscopes pick up signals generated within the body with a chest piece head and deliver them as an acoustic signal to an ear piece via a tube for the clinician to hear. The most common type of electronic stethoscopes use a chest piece head to convert the body's sounds to acoustic, then use microphones to convert the acoustic signal to an analog electrical signal. The analog electrical signal can be processed with analog filters and amplifiers or converted to digital format and processed with digital signal processing techniques.
Another type of electronic stethoscopes uses a piezo (piezoelectric) element to directly convert the internal body signals to analog electrical signals. But using piezo elements creates new challenges. Electronically, a piezo element is modeled as a low impedance voltage source in series with a capacitance. The piezo elements currently in use have a very small equivalent capacitance. This is an important consideration when coupling the piezo element to an amplifier to boost the signal so that it can be heard by the clinician. Unless the amplifier accounts for and compensates for the effects of the series capacitance, the frequency response of the body signal can be altered.
In practice, the capacitance of a coupling cable is significant enough in relation to the capacitance of the piezo element, that it adversely affects the frequency response of the body signal. To avoid the distortions to the signal this could cause, the current state of the art is to put an electronic buffer amplifier in the same housing as the piezo element. The input capacitance of the buffer amplifier is very low and its input impedance is very high, thus preserving the fidelity of the signal from the piezo element. The output impedance of the buffer amplifier is low and relatively immune to the characteristics of the load, including the capacitance and inductance of a filter or cable. The current state of the art for electronic stethoscopes using piezo elements for the pickup sensor requires that a buffer amplifier be co-located in the same housing with the piezo element.
However, by housing the buffer amplifier and the piezo element together, sanitizing the chest piece assembly becomes problematic. Unless the housing assembly is sealed against moisture and liquids, the buffer amplifier electronic components could become damaged while in an autoclave or other liquid sanitizing cleaning method. Sealing the housing with the piezo element and buffer amplifier such that the sensor operation of the piezo element is not adversely affected is difficult and expensive. Therefore, to be competitive in the market, the housing is not perfectly sealed. As a result, these piezo element based electronic stethoscopes are not used in a surgical environment where sanitizing or sterilizing is required.